Rats of various ages were exposed to graded levels of131 I, and randomly selected animals were sacrificed after 1, 3, or 7 days to determine radiation doses to the thyroid glands. The doses were found to vary with age and were not a linear function of administered dose. After four additional months, the remaining rats were injected with a tracer dose of131 I and sacrificed 24 hours later. Radiosensitivity was expressed as the initial radiation dose, in rads, required to reduce incorporation of the tracer to 50% of control levels. On this basis, values of 18,000, 12,000, 5300, and 970 rads were obtained for the initial exposure of adult, weanling, newborn, and prenatal rats, respectively. Alterations in growth and in extrathyroidal iodine metabolism were also noted in these animals. These effects varied with age and dose, but it was not established whether they were direct radiation effects or secondary to the induced hypothyroidism.

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